Geology in space

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A massive cover-up

Venus is our closest neighbour both in terms of distance and size. It is also a very alien world with surface temperatures of 46O °C and clouds of sulphuric acid.

To get an idea of what the surface of Venus looks like we have to use radar to see through the thick clouds and build up a topographic model, and what we see is a world with features which are in many ways similar to our own.

Mountain ranges, showing folded and faulted rocks structures surrounded by large flat plateaus, are very similar to the mountain chains on Earth. Rifts, which are cracks in the surface of the planet along which volcanoes generate new crust are found on both planets. Large volcanoes are found all over the surface.

However whilst the features seem similar on the surface, there distribution there are stark differences: on Earth, these are linked to plate tectonics, rifts form where plates move apart or break up. Mountains form as the crust crumples as these plates collide. There are also deep trenches such as the Mariana trench, linked to subduction as plate sinks down back into the mantle as a recycling of the surface. The only major features not caused by plate tectonics are hotspot volcanoes such as Hawaii. Even these hotspots, show the fingerprints of the movements of plates, which causes them to form chains of volcanoes as the plates move over the source creating lines of islands on the surface of the Earth.

Hawaii islands, formed by the movement of the plates over a hot-spot (JSC)

On Venus, the volcanoes do not form hotspot chains like those on Earth but instead appear at random all over the surface. Whilst both trenches and rifts are seen they are isolated and discontinuous. In short, just like with Mercury, it is a one plate world without plate tectonics.

To understand why this is and why Venus seems to have a universally young surface (even parts of Earth’s surface can be several billion years old) we can look at a side effect of plate tectonics: heat removal. the generation of crust at ridges allows heat to move to the surface and be emitted. Without this plate tectonics, heat builds up under the surface over time. If heat builds up over time, it is possible that a layer of hot rock builds up and the hard crust gets thinner as it warms and softens from the underneath and melt.